Potassium methylsilanetriolate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

7.1 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

4.8 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.48 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.19 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

REACH guidance (ECHA 2012 version 2.1, R.16) states that for substances where a degradation reaction DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the degradation product rather than to the parent itself. TGD (EC 2003a) and REACH guidance (ECHA 2012 version 2.1, R.16) also suggest that when the half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.

As described in IUCLID Section 4.8, condensation reactions of the silanetriol are possible.

Potassium methylsilanetriolate is a mono-constituent ionic substance of 35-55% w/w potassium methylsilanetriolate in aqueous solution that has a very alkaline pH of >12. In more dilute aqueous solutions, the pH will be reduced dependent on concentration. As the pH is reduced, the concentration of the non-ionised form of methylsilanetriolate (methylsilanetriol) increases. At pH <9.0, the substance is no longer in the ionised form and methylsilanetriol is the predominant species at low concentrations, with dimers and oligomers of methylsilanetriol forming at higher concentrations in water. Under conditions relevant for the environment (close to neutral pH and low concentration), the neutral monomer (methylsilanetriol) predominates (>99%).

Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of methylsilanetriol in accordance with REACH guidance for Chemical Safety Assessment.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil REACH Annex VII-X requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

Potassium methylsilanetriolate as sold is a solution of 35-55% potassium methylsilanetriolate in water, with a very alkaline pH of >12, and is fully dissociated to potassium and methylsilanetriolate.

The substance as sold is only stable at high pH (pH>12). When the pH is reduced, the concentration of the non-ionised form (methylsilanetriol) increases. The first pKa of methylsilanetriol is approximately 10. At pH 9.0 and below, the substance is no longer in the ionised form and methylsilanetriol predominates. Methylsilanetriol is a silanol with a methyl side chain and a log K_ow of -2.4 (predicted). Potassium (K+) has been studied extensively in the public literature and is not expected to be hazardous at the concentrations produced during standard environmental studies; it is assessed below. Under comparable conditions of concentration and pH, methylsilanetriolate is equivalent to methylsilanetriol. Therefore, the assessment and the read-across are based on the silanol hydrolysis product, methylsilanetriol.

Potassium methylsilanetriolate, methylsilanetriol and the substance used as a surrogate for read-across are part of a class of low-functionality compounds which act via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the Si atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The silanol hydrolysis products may be susceptible to condensation reactions, see Section 4.8 of IUCLID. The registration substance dissociates rapidly in water and therefore the selection of surrogate substance is based on log K_ow of the resulting silanols and the chemical groups present in them.

Additional information is given in a supporting report (PFA 2013y), attached in Section 13 of the IUCLID 5 dossier.

In the following paragraphs the read-across approach for potassium methylsilanetriolate is assessed for the surrogate substances taking into account structure, hydrolysis rate and physico-chemical properties. Table 1 presents relevant physico-chemical properties and the available ecotoxicological data.

 

Table 1: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number	31795 -24 -1	1185-55-3	115-21-9	2031-67-6
Chemical Name	Potassium methylsilanetriolate	Trimethoxy(methyl)silane	Trichloro(ethyl)silane	Triethoxy(methyl)silane
Relevant silanol	methylsilanetriol	methylsilanetriol	ethylsilanetriol	methylsilanetriol
Molecular weight (parent)	132.23	136.22	163.51	178.31
Molecular weight (silanol)	94.14	94.14	108.17	94.14
log Kow (parent)	n/a	0.7	n/a	2.2
log Kow (silanol)	-2.4	-2.4	-1.87	-2.4
Water sol (parent)	n/a	9.0E+04 mg/l	n/a	2.9E+03 mg/l
Water sol (silanol)	1E+06 mg/l (but will be limited by condensation reactions)	1E+06 mg/l (but will be limited by condensation reactions)	1E+06 mg/l (but will be limited by condensation reactions)	1E+06 mg/l (but will be limited by condensation reactions)
Vapour pressure (parent)	n/a	10680 Pa	47.8 hPa at 20°C	100 Pa
Vapour pressure (silanol)	0.05 Pa	0.05 Pa	0.02 Pa at 20°C	0.05 Pa
Hydrolysis t1/2 at pH 7 and 25°C	Very rapid dissociation under dilute conditions	2.2 h	<1 minute	5.5 h
Hydrolysis t1/2 at pH 4 and 25°C		<0.033 h	<1 minute	0.3 h
Hydrolysis t1/2 at pH 9 and 25°C		0.11 h	<1 minute	0.1 h
Short-term toxicity to fish LC50	n/a	>110 mg/l (results reflect parent substance)	n/a	>500 mg/l   (results reflect hydrolysis products)
Short-term toxicity to aquatic invertebrates EC50	>100 mg/l (nominal; results reflect silanol)	>122 mg/l  (results reflect parent substance)	n/a	>500 mg/l   (results reflect hydrolysis products)
Algal inhibition EC50 and NOEC	n/a	ErC50: >120 mg/l nominal concentration or >3.6 mg/l measured concentration; NOEC:≥120 mg/l nominal concentration or ≥3.6 mg/l measured concentration (results reflect hydrolysis products)	n/a	>500 mg/l   (results reflect hydrolysis products)
Long-term toxicity to fish NOEC	n/a	n/a	≥100 mg/l or ≥45 mg/l as ethylsilantriol	n/a

Read-across from trimethoxy(methyl)silane to potassium methylsilanetriolate:

Under the conditions relevant for the environment (pH 4-9; dilute aqueous solution), methylsilanetriol is the relevant silicon-containing species for the registration substance (see above).

Methylsilanetriol and the surrogate substance trimethoxy(methyl)silane (CAS 1185-55-3) are trifunctional silicon substances containing three Si-O- bonds and one methyl group.

In moist medium, trimethoxy(methyl)silane hydrolyses rapidly (half-life 2.2 hours at pH 7 and 25°C) to methylsilanetriol and methanol.

Methylsilanetriol, the silanol formed when both the registered substance and the surrogate substance are dissolved in water, is the substance on which the chemical safety assessment is based. Therefore read-across from trimethoxy(methyl)silane to Potassium methylsilanetriolate is considered valid.

Trimethoxy(methyl)silane is used to read-across to toxicity to algae endpoint. E(L)C₅₀ values of >120 mg/l (nominal concentration) or >3.6 mg/l (measured concentration) and NOEC values of >=120 mg/l (nominal concentration) or >=3.6 mg/l (measured concentration) have been determined.

Read-across from triethoxy(methyl)silane to potassium methylsilanetriolate:

Under the conditions relevant for the environment (pH 4-9; dilute aqueous solution), methylsilanetriol is the relevant silicon-containing species for the registration substance (see above). Methylsilanetriol and the surrogate substance triethoxy(methyl)silane (CAS 2031-67-6) are trifunctional silicon substances containing three Si-O- bonds and one methyl group.

In moist medium, trimethoxy(methyl)silane hydrolyses rapidly (half-life approximately 5.5 hours 20-25°C, predicted) to methylsilanetriol and ethanol.

Methylsilanetriol, the silanol formed when both the registered substance and the surrogate substance are dissolved in water, is the substance on which the chemical safety assessment is based. Therefore read-across from triethoxy(methyl)silane to potassium methylsilanetriolate is considered valid.

Triethoxy(methyl)silane is used to read-across to short-term toxicity to fish and invertebrates endpoints. E(L)C₅₀ values of >500 mg/l for both species have been determined.

Read-across from trichloroethylsilane to potassium methylsilanetriolate:

The predominant species under environmentally relevant conditions is methylsilanetriol. Trichloro(ethyl)silane is a trichlorosilane with one ethyl group. Trichloro(ethyl)silane reacts very rapidly in water to produce ethylsilanetriol and chlorides. Methylsilanetriol, the relevant silicon-containing species for the registered substance on which the chemical safety assessment is based, and ethylsilanetriol, are both silanetriols, with low MW (94.14 and 108.17 respectively) and similar log K_ow (-2.4 and -1.87 respectively). Therefore read-across from trichloro(ethyl)silane to potassium methylsilanetriolate is considered valid.

Trichloro(ethyl)silane is used to read-across to long-term toxicity to invertebrates endpoint. A NOEC value of >=100 mg/l, or >=45 mg/l as ethylsilanetriol, have been determined.

Considerations on the non-silanol hydrolysis products:

Methanol is well-characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for this substance are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol).

Potassium ions occur naturally (typically at levels 0.1 -5 mg/l in environmental fresh waters). Standard test media contain potassium salts at levels equivalent to approximately 0.4 - 0.5 mg K⁺/l.

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl^-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H⁺concentration) to a level below their tolerable range. Aquatic ecosystems are characterised by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e. g. OECD SIDS for CAS No. 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are considered to be negligible in the environment because the substance and its hydrolysis products will enter the environment through a buffered WWTP before reaching the effluents or will be buffered by the surrounding environment.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because:

·        silanetriols have a predicted first dissociation constants around 10 and so do not significantly affect the pH of an aqueous solution;

·        the silicon-containing hydrolysis products are not toxic to aquatic organisms at 100 mg/l in short-term studies.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values >500 mg/l in fish , >500 mg/l in invertebrates (read across from a structurally related substance, triethoxy(methyl)silane) and >100 mg/l in algae based on read-across from a structurally related substance, trimethoxy(methyl)silane, CAS 1185-55-3). It has reliable NOEC >=120 mg/l in algae. A reliable long-term NOEC of >= 100 mg/l in Daphnia is read across from a structurally related substance, trichloro(ethyl)silane. Under the conditions relevant for the environment (pH 4-9; dilute aqueous solution), methylsilanetriol is the relevant silanol-containing species for the registration substance. Methylsilanetriol is not readily biodegradable. These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP): Acute toxicity: Not classified. Chronic toxicity: Not classified.